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1. Field of the Invention
This invention relates to point-to-multipoint communication; in particular, the invention relates to control of contention for data slots by customer premises equipment in a wireless point-to-multipoint communication system.
2. Description of the Related Art
In a point-to-multipoint network, plural customer provided equipment (CPEs) communicate bidirectionally with a base station controller (BSC) in a cell. Several CPEs can share a single channel for communicating with the BSC. The CPEs and the BSC use time division duplexing (TDD) to facilitate the bidirectional aspect of the communication. The CPEs and the BSC use time division multiple access (TDMA) to facilitate sharing of a channel among plural CPEs.
In TDMA, the BSC grants data slots to the CPEs. Because multiple CPEs share a limited number of data slots, the CPEs and the BSC must negotiate data slot assignments through a process called contention.
In contention, CPEs that have upstream data to send to the BSC first send a short request (REQ) message to the BSC. The BSC responds with a grant of a data slot for use by the CPE.
Problems can arise in contention. First, two CPEs might simultaneously try to use a single request slot to request data slots. This situation is called collision. When collisions occur, neither CPE's request message successfully reaches the BSC. Second, if many request slots are defined so as to reduce a number of collisions and so as to facilitate requests by a large number of CPEs, too much of available bandwidth can be used up by contention traffic. As a result, data throughput can be unacceptably decreased.
One technique used in the prior art to reduce contention traffic is called piggybacking. In piggybacking, a CPE first requests a data slot. When the data slot is granted by the BSC, the CPE sends data packets upstream. The CPE attaches a size of its current data backlog to a header for one of the data packets. As a result, whenever the backlog is non-zero, the BSC knows to grant a data slot to the CPE without the CPE having to send a REQ message. Thus, contention traffic is reduced.
Unfortunately, piggybacking is only effective if upstream traffic from the CPE is of the bursty type, so that the CPE needs to make only one contention request per burst. However, some important traffic sources are non-bursty. Examples of non-bursty traffic include traffic generated by online games and voice sources. Even ordinary Internet traffic running over TCP becomes non-bursty when a communication link is in heavy traffic. In the presence of non-bursty traffic, the piggyback scheme breaks down, and an excessive number of contention requests are sent upstream.